1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display device, in particular, an organic EL display device including a system which enables both correction of a temperature characteristic and correction of screen burn-in.
2. Description of the Related Art
An organic electroluminescence display device (hereinafter, referred to as organic EL display device) has the following characteristics superior to those of a liquid crystal display device. For example, the organic EL display device is not required to include a backlight because the organic EL display device is self-emitting. The organic EL display device has excellent moving-image characteristics with a response time as small as several microseconds. Moreover, since a voltage required for light emission is as low as 10V or smaller for the organic EL display device, there is a possibility of reducing power consumption. Further, in comparison with a plasma display device and a field emission display (FED) device, the organic EL display device is more suitable for reduction in weight as well as in thickness because a vacuum structure is not required.
Each of organic light-emitting diode (hereinafter, referred to as OLED) elements constituting an organic EL display panel corresponding to a screen of the organic EL display device has a temperature characteristic. Even when the same voltage is applied to the OLED element, a current flowing through the OLED element is small at low temperature, whereas the current flowing therethrough is large at high temperature. Therefore, in order to obtain the same brightness, it is necessary to change a power supply voltage depending on the temperature of an external environment. Japanese Patent Application Laid-Open No. 2006-48011 (hereinafter, referred to as Patent Document 1) describes the following technology for detecting temperature fluctuation of the organic EL display panel. According to the technology, the result of detection of a voltage obtained by causing a current to flow from a current source through each of the OLED elements in the panel is subjected to A/D conversion. Then, a voltage of a voltage source for display is changed based on the obtained digital data.
Another problem inherent in the organic EL display device is so-called burn-in. The burn-in is a phenomenon that the OLED element has a lower luminance with elapse of an operation time. A change in characteristics of the OLED element appears as a change in voltage-current characteristic of the OLED element. Specifically, even when the same voltage is applied, the current flowing through the OLED element becomes smaller with elapse of the operation time. A change in characteristics of the OLED element with time differs for each pixel. Therefore, for accurate image display, it is necessary to detect the change in characteristics of the OLED element of each pixel and to feed back the result of detection to an input signal input from a host.
Japanese Patent Application Laid-Open No. 2005-156697 (hereinafter, referred to as Patent Document 2) describes the following technology for allowing the organic EL display panel to perform stable light emission without causing burn-in. According to this technology, the result obtained by measuring the current is subjected to A/D conversion. Based on the obtained digital data, feedback is performed on a driving signal of the OLED element.
With the technology described in Patent Document 1, it is possible to compensate for the effects of the temperature characteristic because the characteristics of the whole organic EL display panel are adjusted by changing the power supply voltage. However, local degradation such as the burn-in cannot be corrected by the technology of Patent Document 1. With the technology described in Patent Document 2, information of the temperature fluctuation generated in the panel cannot be digitally obtained by the AD conversion because the results obtained by the current measurement are compared between neighboring pixels.
A change in voltage of the OLED element is extremely small when the burn-in occurs on the organic EL display panel, whereas a change in voltage due to the temperature fluctuation is large. Therefore, if a voltage range of an analog-to-digital converter included in a system is to cover the temperature fluctuation, a large number of highly accurate comparators are required. As a result, a circuit size is increased to disadvantageously increase the power consumption.